1. Field of the Invention
This invention relates to electronic packaging, and more particularly, to ceramic composites for electronic packaging.
2. Discussion of the Art
Electronic packaging most often refers to an assembly wherein semiconducting silicon chips are attached to a substrate by a conductive pathway and encapsulated by plastic or ceramic materials. Encapsulation with an opaque material is necessary in order to protect the semiconducting silicon chips from light, and hence, the photoelectric effect, which generates stray current. Additionally, the packaging can perform a structural function by protecting delicate wire-bonded leads from damage. Package forms include dual-in-line packages (DIP), chip carriers, flat-packs, and pin grid arrays.
Materials having a low dielectric constant (e.g., less than 4.0) are preferred for substrates for electronic packaging in order to reduce dielectric loss at high frequencies. A loss in signal resolution (wave form shape) can occur in signal transmission when an integrated circuit (IC) chip is coupled to a material having a high dielectric constant. In existing electronic packaging media, materials having a low dielectric constant comprise polymeric materials where both the inherent characteristics of the polymer and a low level of impurities combine to make a material having a low dielectric constant. However, polymers have high thermal expansion coefficients (typically 15.times.10.sup.-6). Because these coefficients do not match those of silicon (4.0.times.10.sup.-6), these materials suffer from breakage due to thermally-induced mechanical stresses between the substrate and the silicon chips. They also suffer from high dissipation loss factors. Materials for high performance electronic applications are preferably those in which the dielectric constant approaches that of air (1.0), so that capacitive effects will be minimized, impedances will match, and signal transmissions will not be delayed.
Ceramics are used as substrates for integrated circuits, and they can form the housings for integrated circuit assemblies. For hermetic sealing of integrated circuit assemblies, ceramics are often the only suitable materials because they can withstand higher temperatures and humidities than can polymeric materials. Ceramics are desirable for their high temperature stability, adjustable thermal expansion coefficients, and controllable thermal conductivities.
The use of ceramics in electronic packaging applications is well-known. See, for example, L. M. Levinson, "Electronic Ceramics", p. 1-44, Marcel Dekker, Inc., N.Y. and R. R. Tummala, "Ceramics in Microelectronic Packaging", Am. Ceramic Soc. Bull., 67 (4) 752-58, 1988. Addition of glass to alumina to form glass-ceramic substrates having a dielectric constant of approximately 5.6, which substrates are capable of being sintered with copper or gold, is known. See, for example, Y. Shimade, et al., "Low Firing Temperature Multi Layer Glass-Ceramic Substrate", IEEE Trans. Compon. Hybrids Manuf. Technol., 6 (4) 382-86, 1983.
EP 234896 discloses low dielectric constant material for use as a thick film in very large scale integrated (VLSI) devices. The material comprises a thick film insulation matrix, a thick film organic vehicle, and hollow silica glass microspheres for use in the formation of thick film circuits. The low dielectric material is screen printed, dried, and fired. Dielectric constants ranging from 3.5 to 4.5 can be obtained.
A composite material having a dielectric constant of 4.0 has been reported by Y. Iwata, et al., in "Development of Ceramic Composite Porous Ceramic and Resin Composite with Copper Foil", pp. 65-70, in International Microelectronic Conferences, ISHM, Reston, Va., 1986. This was achieved by the use of cordierite, which has a dielectric constant varying from about 4.5 to about 6.0, impregnated with a low dielectric constant epoxy.
The primary use of ceramic packages are in high performance applications wherein a large number of circuits must be wired on individual ceramic substrates and hermetically sealed. The high performance requirement is that signal delay be at a minimum. The speed of the signals through the substrate is determined by the distance a pulse must travel and by the dielectric constant of the medium through which it travels, and is expressed in the following equation: ##EQU1## wherein T.sub.d represents the time delay in nanoseconds, L represents the distance a signal must travel in inches, K represents the relative dielectric constant of the ceramic, and C represents the speed of light in inches per second. A minimal time delay can be obtained by the use of ceramic material of lower dielectric constant, which can be made of materials having low electronic, dipole, and ionic polarizabilities, low molecular weight (as the dielectric constant at very high frequencies is proportional to the atomic number), low bond strength, and low density.
To improve the performance of electronic packages, the trend is to reduce their size, thereby increasing the speed of the electronic signals they carry. These characteristics can be achieved by an increase in density of silicon chips in integrated circuits (IC). However, this increase in packaging density results in generation of additional heat. Accordingly, substrates and housings for electronic packaging must be able to withstand, and preferably to dissipate, this heat.
Glass bubble-organic matrix composites are generally undesirable for high performance electronic packaging applications because they cannot withstand the high temperatures required in firing the metallized circuit layer. Also, they have high thermal expansion coefficients (e.g., in excess of 10.times.10.sup.-6 /.degree.C.). Ceramic composites containing silica glass bubbles deform at temperatures above 800.degree. C., because of large volume expansion due to silica phase transformation. See, for example, Verweij, et al., "Hollow Glass Microsphere Composites: Preparation and Properties", J. Matl. Science, 20, 1985, pp. 1069-78. In addition to degradation, hollow glass microspheres, especially soda lime glass compositions, produce high dielectric loss factors, which are undesirable as electronic substrates in IC packages.
It would be desirable to develop a ceramic that can be fired at the high temperature required for metallization and that would also have a dielectric constant of no greater than 5.0, in order to maintain the propagation velocity of high-speed digital signals in circuits. It would also be desirable to develop a ceramic having thermal expansion coefficient matching that of silicon chips. It would also be desirable to develop a machinable ceramic which would offer transparency to microwaves and radiowaves.